As I said in your other thread, I highly recommend giving them access to soil from day one, so they are exposed to the microbes naturally. Giving them concentrated droppings I would not do. The idea is to expose them gradually, not give them a ton all at once. Poopy water is NOT healthy for baby chicks. Scratching around, pecking, and dustbathing in soil IS. Poopy water is not healthy for anybody or any thing.
I have a Brinsea Ecoglow 50, and I usually brood between 6 - 20 chicks at a time. I think that it would be pretty crowded for the 50 chicks it's supposed to be adequate for, so I would go larger if you can. And chicken math usually strikes us all at some point.....I always go bigger if I can. The day just may come when you think you are going to get 6 chicks and you end up with a dozen or more, because, well, just because! I love my ecoglow, the chicks have always done well with it. Besides being a safe heat source that you don't have to worry about, it also satisfies their instinct to hide under mama. They sleep at night just like they would with mama since there is not any extra light from the heat lamp. I set mine up on a slant, so one side is a bit lower than the other so they can all find their comfortable spot closer or farther from the plate as needed. If you find them mostly around the edges, then usually raise it up just a bit. And the screw in legs on the 50 make height adjustments very easy. None of mine have ever bothered the power cord at all.